Swine Flu Jumps Back to Pigs and Keeps Evolving

A new strain of swine flu shows that the pandemic version has jumped from humans back to pigs, where it’s evolving in new and unpredictable ways.

The new strain, identified in a Hong Kong slaughterhouse, isn’t especially virulent. But the findings emphasize the need for continued vigilance. Swine flu isn’t going away.

“Hoping for the best while preparing for the worst seems a sensible strategy,” said University of Hong Kong virologist Malik Peiris. The strain is described June 18 in Science.

Peiris and study co-author Yi Guan are best known for identifying the SARS virus and helping to contain its 2003 outbreak. They were also among the researchers who identified swine flu’s origins after its outbreak last year.

Technically known as H1N1/2009, swine flu’s popular name hinted at its ancestry: a fusion of two swine strains, one from Asia and another that had erupted in the 1990s on U.S. factory farms.

Swine flu spread rapidly around the world, infecting tens of millions of people, and exposing profound flaws in the ability of governments and companies to handle pandemic flu. Fortunately, the strain was far less virulent than originally feared, killing about 19,000 people.

But virologists warned that H1N1/2009 was now a major new player in the global flu scene, and would continue to evolve — not just in humans, but once again in pigs, who proved susceptible to the pandemic strain. That’s exactly what the latest findings show.

“Everyone talks about viruses that go from animals to people, but it’s a two-way street. We reintroduce them to animals, where they reassort and become pathogenic,” said Columbia University epidemiologist Ian Lipkin, a member of the World Health Organization’s virus surveillance network. Lipkin was not involved in the study.

Peiris and Guan found the new strain in January in a Hong Kong slaughterhouse, where they regularly sample pigs arriving from farms in southeast China. It contains a gene from the pandemic swine flu, plus genes from the two strains that originally mixed to create the pandemic flu.

Having sequenced the new strain’s genes, the researchers recreated it in a laboratory and exposed it to pigs. The strain proved contagious but only mildly virulent.

In their surveillance efforts, the researchers have not found evidence of swine flu recombining with H5N1 avian influenza, an eventuality that Guan once said would prompt him to “retire immediately and lock myself” in a high-security lab. That’s reassuring, but Peiris warned against complacency.

“If this pandemic virus was marginally more virulent than it turned out to be, the discussion right now would have been [about] why the pandemic preparations failed to protect the population, rather than why global health authorities over-reacted,” said Peiris.

“Flu is unpredictable. People have been saying that we should expect a new pandemic every 30 years, and we had one last year, so we can rest. That’s not true,” said Lipkin. “This virus evolves rapidly. We have to stay vigilant.”

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